Radiation sources for semiconductor lithography usually contain collector optics for collecting and focusing the radiation divergently emitted by a narrowly circumscribed, but not point-shaped, source location into a concentrated beam bundle along an optical axis. The beam bundles may proceed from a discharge-produced plasma (DPP) or a laser-produced plasma (LPP). The beam bundle is usually focused on a defined aperture in an intermediate focus (IF) to provide it as radiation source location for a specific application (e.g., for a scanner for semiconductor lithography).
For this purpose, it is necessary to align the optical axis of the collector optics within the source module relative to the axis defined through the position of the source location and of the adjoining optical system of the application. These two axes must be brought into correspondence and pass through the center of the IF aperture. Otherwise, a portion of the beam bundle would be reflected at the IF aperture diaphragm of the radiation source and the transmitted beam bundle would therefore be cut off. At the same time, besides the total intensity, the homogeneity within the cross section of the beam bundle should also be maximized after the intermediate focus.
However, the difficulty in the required alignment of axes consists in that the installation location of a metrologic measurement module is severely limited and in that, owing to the fact that the application (e.g., a lithographic scanner) follows directly, there is no free space behind the intermediate focus without completely decoupling the radiation source module from the application or interfering with the beam path within the application. Both of these options are undesirable.
Radiation sources for generating soft X-ray radiation (EUV) are operated at low pressures of a few pascals (1 . . . 30 Pa) and, within a vacuum chamber, a plasma generating module generates either a discharge-produced plasma (DPP) or a laser-produced plasma (LPP) and produces a concentrated radiation source location (i.e., a plasma that is spatially extensive but point-shaped in a first approximation). The radiation which is emitted isotropically by this source location is imaged by collector optics in an intermediate focus (IF) in the immediate vicinity of an exit aperture (IF aperture) of the radiation source or of the vacuum chamber used for this purpose. The IF aperture often constitutes the interface between the radiation source module (including collector-condenser optics) and a downstream optical system of the application.
The quality of the beam bundle supplied in the intermediate focus is crucial for the application downstream thereof; for this reason, an optimal adjustment and simple correction of the alignment of the radiation source location (plasma), collector-condenser optics (hereinafter abbreviated as collector optics), and IF aperture are of the utmost importance within a radiation source module. Adjusting means for aligning the collector optics are commonly provided in the radiation source module between the radiation source location and IF aperture, allowing the collector optics to move its six degrees of freedom.
The conditions for the alignment of the beam bundle are defined for two planes: the plane of the exit aperture of the source module (i.e., the aperture of the intermediate focus) and a far-field plane behind the intermediate focus. In addition to the correct alignment of the beam bundle relative to the optical axis of the application unit, these conditions also include the intensity at a desired wavelength and the uniform distribution thereof over the cross section of the beam bundle. Accordingly, checking and correction of the alignment of collector optics and source location (plasma) requires data that are supplied by a measuring device which allows the characteristics of the beam bundle for a far-field plane to be monitored dependably in a stable manner over the long term and which acquires these measurement data without interfering with the beam path of the application unit. In this regard, however, it is problematic that the characteristics of the beam bundle in front of the aperture differ from those behind the aperture because, for example, a portion of the radiation does not pass the aperture.